Demographics are "a statistical view of a population, generally including age, gender, income, schooling, occupation and so on" (source).
You'll notice overlap in the data sources for demographic data (this page) and psychographic data (next page). That's because these data types go hand-in-hand to help you develop a fuller picture of your potential market.
The U.S. government is the single largest collector of demographic data and statistics on business and the economy in the United States, through agencies such as the Census Bureau, the Bureau of Labor Statistics, and the Bureau of Economic Analysis. This data is freely available online. Here are just a few quick links:
PolicyMap is an online mapping tool with data on demographics, real estate, health, jobs and more in communities across the U.S. The Public Edition provides access to all of the public data available in the platform. You'll need to create a free account. (UNC affiliates with an active ONYEN have full access to PolicyMap through the UNC Libraries.)
Other people in your industry are also interested in discovering qualities of potential customers. (Who cares?)
Since you may have already found at least one relevant industry association, investigate what content exists on customer demographics.
You must have an active ONYEN and password to access these databases.